Various gain control techniques are known for radio frequency and intermediate frequency amplifiers. As the use of portable communications equipment has increased (e.g., cellular telephones), it has become important to implement gain control techniques using a minimum of circuitry to reduce device cost and weight.
In certain applications, it is important to be able to predict the gain of an RF amplifier strictly through knowledge of a control signal. Gain, in such circuits, is generally expressed in decibels (dB) as follows: EQU Gain=(a)log(CONTROL)+b (1)
Where:
CONTROL is measured in volts or amperes; and a and b constants.
Linear gain control in such circuits has been implemented through the use of a multiplier that employs a differential transistor amplifier structure. Such an arrangement requires many matching transistors and is practical only when implemented as a monolithic integrated circuit. Others have implemented linear gain control by the use of a compensation circuit which, in response to a control signal, modifies the transfer characteristic of an RF amplifier so as to negate its nonlinearities. Such a circuit is shown in U.S. Pat. No. 5,099,204 to Wheatley, III.
A variety of prior art amplifiers employ PIN diodes for gain control. A PIN diode is a three layer device consisting of an intrinsic high resistance layer in the center, with conducting P and N layers on either side. The intrinsic layer's RF resistance can be controlled by a DC bias current and is approximately proportional to the inverse of the current: ##EQU1## Where K and .varies. are constants; and I.sub.DC is the dc bias current
"Communications Receivers", Ulrich L. Rohde, T. T. N. Bucher, McGraw-Hill, 1988, pages 235-41 describe various applications of PIN diodes for amplifier gain control. In one version (page 237), a pair of PIN diodes controls the impedance of a common emitter circuit in an emitter coupled RF amplifier. In another version (page 239), three PIN diodes are employed as a variable attenuator between amplifier stages. In "Practical Variable Gain Amplifiers", Franklin, RF Expo West '92, pages 41-54, a number of PIN diode attenuators are shown used as variable gain controls for RF amplifiers. A number of diode attenuator topologies are disclosed, including series, shunt, Tee, bridge and bridge/Tee configurations. Similar configurations are employed in commercially available RF amplifiers, e.g., the AGC-330, Voltage-Controlled AGC Amplifier, marketed by Avantek, 481 Cottonwood Drive, Milpitas, Calif. 95035. The AGC-330 Amplifier employs a double-T arrangement wherein the PIN diodes are separated by a resistor in the signal feed path.
In all of the above noted gain control configurations, known PIN diode characteristics are employed to provide gain control functions for RF amplifiers without specific consideration of gain control linearity. They do not take into account the affect of complex impedances in the circuit that vary the gain control effect of the PIN diode (or diodes). Those impedances cause the gain of an amplifier to vary in other than in a linear fashion in response to an applied control voltage.
Accordingly, it is an object of this invention to provide a linear gain control circuit for an RF amplifier.
It is a further object of this invention to provide a linear gain control circuit that operates under conditions of complex impedance variations within a narrow band of applied RF frequencies.
It is another object of this invention to provide a linear gain control circuit for an IF amplifier that is both simple and requires no special matched devices.